The present invention relates to a method of manufacturing a semiconductor device comprising refractory metal silicide contacts to source/drain and silicon gate regions. The present invention has particular applicability in manufacturing reliable high density semiconductor devices with submicron design features, shallow junction depths and cobalt silicide contacts to source/drain regions.
Current demands for high density and performance associated with ultra large scale integration require design rules of about 0.18 microns and under, increased transistor and circuit speeds and improved reliability. As device scaling plunges into the deep sub-micron ranges, it becomes increasingly difficult to maintain performance and reliability.
In the manufacture of conventional complementary metal oxide semiconductor (CMOS) devices, referring to FIG. 1A, isolation regions 110, called field oxide regions, are formed in a semiconductor substrate 100 of silicon dioxide by local oxidation of silicon (LOCOS) or by shallow trench isolation (STI). A conductive gate 130, such as polysilicon, is also formed on substrate 100, with a gate oxide layer 120 in between. Dielectric spacers 140 are formed on sidewalls of the gate 130, and source/drain regions 150 are formed on either side of gate 130 by implantation of impurities.
As gate lengths are reduced to below 0.5 xcexc, refractory metal silicide layers, such as titanium silicide, are typically formed over source/drain regions 150 and gate 130 to reduce the sheet resistivity of these components, thereby improving device performance. Referring to FIG. 1B, a titanium layer 160 is deposited, as by sputtering, over the entire substrate 100 as well as field oxide 110, gate 130 and spacers 140. A low temperature rapid thermal anneal (RTA) reaction creates a first-phase titanium silicide (C49) on the exposed silicon of gate 130 and source/drain regions 150. The unreacted titanium over field oxide 110 and spacers 140 is then removed, and a high temperature RTA reaction changes the first-phase titanium silicide into a low-resistivity second-phase titanium silicide 170 (C54), as shown in FIG. 1C. Since the titanium silicide does not form on field oxide 110 or spacers 140, it is self-aligned to the gate 130 and source/drain regions 150. Hence, the titanium silicide formed in this process is known as xe2x80x9ctitanium salicidexe2x80x9d (self-aligned silicide).
Titanium salicide is effective in decreasing sheet resistivity if the gate length is greater than about 0.25 xcexc. At a gate length of about 0.25 xcexc the titanium silicide sheet resistance rises dramatically due to narrow-line effects; that is, the low-resistivity silicide C54 does not completely form because first-phase C49 grains are very large (about 0.5 xcexc), and hence it is not possible to fit enough grains on the gate to nucleate and grow a sufficient amount of C54.
To maintain low sheet resistance as gate lengths are decreased in scale below about 0.25 xcexc, cobalt is typically used instead of titanium in silicide formation. Cobalt silicide does not display the undesirable narrow-line effects of titanium silicide because the conversion from its first-phase cobalt silicide to its low-resistivity second-phase cobalt silicide is a diffusion reaction, rather than the nucleation and growth reaction of titanium silicide, and therefore the relationship of grain size to gate size is not a limiting factor.
However, the cobalt salicide process has a drawback in that cobalt silicide is more likely than titanium silicide to cause source and drain junction leakage, which can result in unacceptably high power dissipation as well as functional failure. This problem becomes especially critical as gate lengths are scaled below 0.25 xcexc, and source and drain junctions are typically made shallower to prevent transistor short-channel effects. Since shallow junctions are more susceptible to junction leakage than deep junctions, cobalt silicide related junction leakage effectively limits CMOS device scaling.
A cause of this junction leakage, referring to FIG. 2, is the unevenness of the interface between the cobalt silicide 210 and the silicon source/drain regions 220, which results in an insufficient distance between portions of the bottom of the cobalt silicide 210 and source/drain junctions 220a. When a junction 220a is biased, a depletion region (i.e., an area depleted of free carriers) is formed which extends on either side of the junction 220a. Since the distance the depletion region spreads from the junction 220a is inversely proportional to the doping of the region, and source/drain region 220 is more heavily doped than substrate 200, the depletion region spreads mainly into substrate 200. Nevertheless, if cobalt silicide 210 extends into the depletion spread, leakage can occur as carriers are swept across this highly charged region.
Junction leakage also occurs due to consumption of silicon of substrate 200 during silicide formation. For example, when cobalt silicide is formed to a given thickness, a thickness of substrate silicon slightly greater than the thickness of the silicide is consumed. As junctions become shallower with device scaling, consumption of substrate silicon during silicidation results in an insufficient distance between portions of the bottom of the cobalt silicide 210 and source/drain junctions 220a and, hence, junction leakage. Junction integrity can be maintained by providing a large enough distance between junction 220a and the interface of silicide 210 and source/drain region 220; i.e., by reducing the thickness of cobalt silicide 210. However, reducing its thickness increases the sheet resistivity of cobalt silicide 210, thus reducing its effectiveness.
Copending U.S. patent application Ser. No. 09/187,427 and U.S. Pat. No. 6,096,399 disclose methodologies for forming junctions self-aligned to the bottom of the cobalt silicide, thus avoiding junction leakage by addressing the problem of insufficient distance between portions of the bottom of the silicide and the source/drain junctions. According to these methodologies, a doped film is deposited on top of the cobalt or cobalt silicide, from which impurities are diffused through the cobalt silicide to form source/drain regions having a junction depth which is substantially evenly spaced from the metal silicide/silicon interface.
Copending U.S. patent application Ser. No. 09/318,824 addresses the problem of substrate silicon consumption during silicidation, facilitating the formation of shallower source/drain junctions demanded as devices are further scaled. According to this methodology, a doped amorphous silicon film is deposited on top of the cobalt. The amorphous silicon film is consumed during silicidation, thereby decreasing the amount of substrate silicon consumed, and the dopant is diffused through the cobalt silicide to form source/drain regions having ultra-shallow junctions substantially evenly spaced from the metal silicide/silicon interface.
Disadvantageously, when implementing the methodologies of U.S. Pat. No. 6,096,599 and copending applications Ser. Nos. 09/187,427 and 09/318,824, the impurities may diffuse, upon heating, out of the doped film and into the environment rather than into the substrate. This may occur whether the diffusion step is carried out at the low temperature RTA step, the high temperature RTA step, or at a separate heating step. As a result of unwanted out-diffusion, the amount of impurities available to dope the substrate may be insufficient to properly form the source/drain regions.
There exists a need for a method of manufacturing a semiconductor device by diffusing impurities from a doped layer into the substrate during or after silicidation to form source/drain regions, without a loss of dopant due to out-diffusion of impurities to the environment.
An advantage of the present invention is a method of manufacturing a semiconductor device having source/drain regions formed by diffusion of impurities from a doped layer during or after silicidation which avoids out-diffusion of impurities from the doped layer to the environment.
Additional advantages and other features of the present invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by a method of manufacturing a semiconductor device, which method comprises forming a metal layer on a main surface of a semiconductor substrate containing silicon;
heating to form a layer of metal silicide; forming a doped layer having impurities on the metal layer or the metal silicide layer; and heating to diffuse the impurities into the substrate to form source/drain regions having a junction depth below the metal silicide layer, in an environment that substantially prevents out-diffusion of the impurities from the doped layer to the environment.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.